1. Field of the Invention
The present invention relates to a process for treating a gas containing a fluorine compound with the main object of producing aluminum fluoride from the gas containing the fluorine compound in a high yield for a long period while maintaining a stable state.
2. Description of the Prior Art
In the case where a gas containing a fluorine compound, particularly a gas of low concentration of the fluorine compound such as exhaust gases from electrolytic furnace for aluminum production, which contain gaseous fluorine compounds, such as hydrogen fluoride, dihydrogen difluoride, and carbon fluoride, particulate (i.e. dusty) fluorine compounds such as cryolite and aluminum fluoride, as well as other gaseous and particulate compounds such as carbon dioxide, sulfur dioxide, alumina and carbon, and the like is treated by absorption with only water, not only the efficiency of absorption at the gas-liquid equilibrial surface is low but also it has been technically difficult to recover fluorine as aluminum fluoride because the concentration of fluorine in the thus obtained aqueous absorbing solution is low. As a countermeasure for the above-mentioned demerit, in the case where an absorbent including an aqueous solution containing water-soluble aluminum salt is used instead of water, the efficiency of absorption is improved and the concentration of fluorine in the absorbent is raised with the result of showing a considerable technical progress since the fluorine component in the gas is caught by the reaction with aluminum compound. However, as will be shown below, the industrial value of such a countermeasure is not necessarily high enough.
Namely, the process described in Japanese Patent Publication No. 28289/70 (hereinafter referred to as the first prior art) is a process wherein an aqueous solution containing aluminum sulfate and the like is used as an absorbent. According to the examination of the present inventors, since the concentration of sulfate radical in the above-mentioned absorbent is as high as 5 to 8% by weight, a part of or almost all of fluorine has come to be water-soluble salt such as Al.sub.2 F.sub.4 SO.sub.4 and the like in the absorbing step. Accordingly, although the eduction and adhesion of solids hardly occur, the countermeasure has demerits of taking a long period for the production of aluminum fluoride hydrate and the like in the producing step of solid phase and hence of difficulty in industrialization.
Another process described in Japanese Patent Publication No. 21399/78 (hereinafter referred to as the second prior art) uses an aqueous solution containing aluminum fluoride or an aqueous solution containing aluminum fluoride and alumina or aluminum hydroxide as the absorbent for a gas containing a fluorine compound. According to the examination by the present inventors, since this absorbent does not contain sulfate radical, or even in the case where SO.sub.x (sulfur oxides) in the exhaust gas from electrolytic furnaces for aluminum production is considered, the concentration of SO.sub.x is ordinarily as low as 0.003 to 0.01%, and even in the case where the concentration of SO.sub.x is still higher, SO.sub.x is hardly absorbed by such an absorbent owing to the operating conditions, thus the concentration of sulfate radical in the absorbent is negligibly small normally as less than 0.1 g/l, hence water-soluble salts such as Al.sub.2 F.sub.4 SO.sub.4 and the like are not produced in the absorbing step. Accordingly, the demerit of the first prior art of taking a long period for producing the solid phase has been solved by the second prior art, and in this case of the second prior art, the reaction of producing the solid phase proceeds easily. However, there is another side to the second prior art of the inevitable eduction and adhesion of solid matter in the absorbing step. Although it is possible to reduce the occurrence of eduction and adhesion of solid matter to a certain extent by a countermeasure of reducing the amount of addition of aluminum hydroxide or alumina to the absorbing solution to less than the theoretical amount, namely, by increasing the acidity of the absorbent so as to form acidic aluminum fluoride represented by the formula: H.sub.n AlF.sub.3+n wherein 0&lt;n.ltoreq.3 in the solution, there is a demerit that it is difficult to operate the system under such consitions for a long period in a stable state.
As has been stated above, neither the first prior art nor the second prior art can be said to have been completed as an industrial process, besides with a very few instances of actual installation.